This invention relates to a method and apparatus for eliminating television interference and more particularly to a two terminal electronic filter useful in minimizing TV interference from adjacent noncommercial FM radio stations.
Due to present limitations now placed on the allocation of frequencies for commercial and noncommercial broadcasting stations, there arise many difficulties in the physical location of educational FM stations and television stations. One of the primary difficulties is the interference caused by educational broadcast stations operating at the low end of the noncommercial FM broadcast band, and which therefore lies immediately adjacent the upper end of the television channel 6 television band. Actual or potential interference to the color subcarrier within the channel 6 band imposes, generally, a requirement for the physical separation of the respective broadcast stations, and hence prevents the full development of public radio and specifically the utilization of educational FM radio broadcasting in areas served by television channel 6 stations. The problem arises due to the inadequate selectivity of present home television receivers--improved receiver selectivity is a long-range solution, if adopted and enforced, and hence offers no immediate or short-term solution. Co-location of the respective stations simplifies a solution to the problem, but frequently is neither feasible nor practical, or simply is not adopted. Expensive filters providing adequate rejection are available, but are not susceptible of wide-spread use; frequently, such filters as are adequate electronically must be installed by trained personnel, and again do not offer a solution to the problem which is either practical or feasible. Most presently available filters, on the other hand, either provide insufficient attenuation of the FM signal, or, conversely, if providing sufficient attenuation, have too broad an attenuation characteristic with the result that the insertion loss over the frequency range encompassed by TV channel 6 is unacceptable. There is thus a substantial need for low cost, immediately available apparatus by which such interference may be reduced so as to permit the licensing of educational broadcasting stations physically adjacent the television channel 6 stations.
Central to the elimination of this interference problem is the development of a low cost FM band rejection filter which can be easily installed across the 300 ohm input terminals of a standard television receiver, which filter is effective in reducing the level of FM interference to the reception of television channel 6, and can be installed by non-skilled persons, such as the typical owner/user of a TV receiver. In general, this filter must achieve at least a 20 dB rejection at the frequency of the interfering FM stations, e.g., a frequency (90 MHz.) within the noncommercial FM band (88.1-91.9 MHz.) with no more than a 3 dB insertion loss at the channel 6 color subcarrier frequency (86.85 MHz.). Moreover, there must be negligible insertion loss at all VHF TV channels 2 through 13.
Multiple pole-zero, four-terminal filter networks exist which, while providing a very high degree of rejection, have an associated attenuation characteristic far too broad with respect to the insertion loss over the frequency range encompassed by television channel 6. Additionally, it will be appreciated that a four-terminal device typically is expensive and is not readily adapted for installation or adjustment by the user of the television.
It is accordingly an object of this invention to provide a novel two terminal electronic filter network of simple configuration and low cost, which may be installed directly to the antenna terminals of the television receiver without requiring cutting of the television antenna lead-in.
It is another object of this invention to provide a novel low cost FM band rejection filter to reduce the level of FM interference from noncommercial FM stations to the reception of television channels.
It is yet another object of this invention to provide a novel, low cost two terminal electronic filter network for connection across the antenna input terminals of a television receiver to eliminate interference from signals in the noncommercial FM frequency band adjacent to the television channel 6 band, in which attenuation at network resonance is reduced to a predetermined minimum sufficient to achieve adequate rejection of interference from the FM frequency band, while lowering insertion loss at TV channel 6 and all other VHF channels also to acceptable levels.
It is a still further object of this invention to provide an electronic filter network for application across television receiver input terminals to provide at least 20 dB input signal rejection at the resonant frequency thereof while providing no more than a 3 dB insertion loss for the television channel 6 color subcarrier and a negligible insertion loss at the VHF television channels 2 through 13.
It is yet another object of this invention to provide a novel method of reducing interference from educational FM broadcasting to TV channel 6 and all other VHF television channels 2 through 13, while assuring acceptable levels of insertion loss at the frequencies of those TV channels, through use of the novel filter network of this invention.
These and many other objects and advantages of the present invention will become apparent to one skilled in the art to which the invention pertains from a perusal of the following detailed description when read in conjunction with the appended drawings.